“The bacteria found are opportunistic pathogens - the source of human disease.

“These should not be present at any level - never mind the significant numbers found.”

Cleanliness of tables, trays, and high chairs at the chains was also tested at 30 branches.

A Starbucks spokesman said it takes hygiene “extremely seriously” and “nothing is more important” than customer safety.

“We have moved quickly to conduct our own investigation into the claims about the stores,” he added.

“All employees nationwide have received updated training on our high standards of hygiene including ice handling.”

Costa said: “We were disappointed with the findings, especially as these stores are all rated ‘very good’ with a the top hygiene rating of five.

“Following these results we took immediate action to review our food safety procedures and have updated our ice-handling guidelines and are in the process of introducing new ice equipment storage across our estate.”

A spokesman for Caffe Nero described the findings as concerning and added: “A thorough investigation is now under way and the appropriate action will be taken.”

“99% of our stores are rated as very good, good or satisfactory by their local EHO (environmental health officer) - making us one of the most highly rated businesses on the high street,” he said.

The first episode of the new Watchdog programme will air on BBC One on Wednesday at 8pm.